Filters are known to provide attenuation of signals having frequencies outside of a particular frequency range and little attenuation to signals having frequencies within the particular frequency range of interest. As is also known, these filters may be fabricated from ceramic materials having one or more resonators formed therein. A ceramic filter may be constructed to provide a lowpass filter, a bandpass filter or a highpass filter, for example.
For bandpass filters, the bandpass area is centered at a particular frequency and has a relatively narrow bandpass region, where little attenuation is applied to the signals. For example, the center frequency may be at 750 Mega Hertz (MHz) with a bandpass region of less than 2 MHz. While this type of bandpass filter may work well in some applications, it may not work well when a wider bandpass region is needed or special circumstances or characteristics are required.
Block filters typically use an electroded pattern printed on an outer (top) surface of the ungrounded end of a combline design. This pattern serves to load and shorten resonators of the combline filter. The pattern helps define coupling between resonators, and can define locations of transmission zeroes.
These top metallization patterns are typically screen printed on the ceramic block, which can be difficult and time consuming in the manufacturing process. Many block filters include chamfered resonator through-hole designs to facilitate this process by having the loading and coupling capacitances defined within the block itself, to facilitate and simplify the manufacturing process. The top chamfers help define the intercell couplings and likewise define the location of the transmission zero in the filter response. This type of design typically gives a response with a low side zero. To achieve a high side transmission zero response, chamfered through-holes are placed in the grounded end (bottom) of the ceramic block filter. Thus, a high zero response ceramic filter would typically have chamfers at both ends of the dielectric block. A double chamfer filter is more difficult to manufacture because of the tooling requirements, and precise tolerances and required structure in making double chamfered through-holes at the top and bottom surfaces of the filter.
A bandwidth of a filter can be designed for specific passband requirements. Typically, the wider the passband, the lower the insertion loss, which is an important electrical parameter. However, a wider bandwidth reduces the filter's ability to attenuate unwanted frequencies, typically referred to as the rejection frequencies. The addition of a transmission zero in the transfer function at the frequency of the unwanted signal could effectively improve the performance of a ceramic block filter, as detailed below.
A ceramic filter which can be easily manufactured to manipulate and adjust the frequency response, preferably with a high side zero, to attenuate unwanted signals, could improve the performance of a filter and would be considered an improvement in ceramic filters.